Borane- and Silylium-Catalyzed Difunctionalization of Carbohydrates: 3,6-Anhydrosugar Enabled 1,6-Site Selectivity

Clarke, Joshua J.; Basemann, Kevin; Romano, Neyen; Lee, Stephen J.; Gagné, Michel R.

doi:10.1021/acs.orglett.2c01243

Cited by 3 publications

(3 citation statements)

References 37 publications

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“…The observation that the BAr 3 -initiated reaction is significantly faster than the TMS-OTf catalyzed reaction suggests the importance of weakly coordinating fluorinated triarylborate counterions (e.g., 37 ) formed in situ . The question of whether boranes act as catalysts or as initiators has been raised before, and both computations and experiments have indicated that B(C 6 F 5 ) 3 ( 6 ) is more often an initiator than a catalyst, e.g., in group transfer reactions. In that context, our study on borane-initiated conjugate addition reactions highlights that the Lewis acidity of an initial Lewis acid catalyst does not necessarily relate to its catalytic activity in a reaction, and other factors like the Lewis acidity or electrophilicity of species formed in situ or the coordinating ability of intermediates are of crucial importance.…”

Section: Discussionmentioning

confidence: 99%

“…In previous studies on 1,2-additions (Scheme A), neither the change of the catalytically active species during the reaction has been directly observed nor have the kinetics of individual reaction steps been evaluated. − Since competing metal vs silylium activation also arises in Lewis acid-catalyzed conjugate additions , and has also been reported with borane catalysts in other precedents, , we have now performed an extensive mechanistic investigation of the borane-initiated 1,4-additions of allylsilanes and silyl enol ethers to α,β-unsaturated carbonyl compounds, the electrophilic reactivities of which have previously been quantified by us …”

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confidence: 99%

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Resolving the Mechanistic Complexity in Triarylborane-Induced Conjugate Additions

et al. 2022

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Previous studies have shown that the catalytically active species in the Lewis acid-catalyzed addition reactions of allyl silanes or silyl enol ethers to carbonyl compounds or Michael acceptors are often silylium-carbonyl adducts rather than the adducts of the carbonyl group with the Lewis acid used for the induction of the reaction. Indirect evidence for such catalyst variations has so far been derived from double-label crossover experiments and comparisons of absolute reaction rates. We have now performed a detailed investigation on the kinetics and mechanism of the triarylborane-initiated conjugate addition reactions of allylsilanes and silyl enol ethers to α,β-unsaturated carbonyl compounds. NMR spectroscopic monitoring of such reactions gave rise to sigmoidal kinetic profiles, allowing us to directly follow the change of the catalytically active Lewis acid from triarylboranes in the induction period to silylium ions during the main part of the reaction. Crossover experiments and the isolation of four-and five-membered cyclic intramolecular trapping products provided further insight into the mechanism. DFT calculations of various mechanistic variants and kinetic modeling elucidated the operation of a complex reaction network, which rationalizes the experimental observations.

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Section: Discussionmentioning

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Resolving the Mechanistic Complexity in Triarylborane-Induced Conjugate Additions

et al. 2022

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“…The development of chemoselective reduction reactions is an important objective in organic synthesis. 1,2 The ability to diversify oxidized precursors into distinct products or to control the extent of reduction in systems with multiple sites of reactivity can streamline syntheses by avoiding unnecessary redox adjustments. In particular, the chemoselective conjugate reduction of enones plays an important role in the synthesis of aliphatic ketones, which are prevalent in biologically active compounds.…”

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Hemiboronic Acid-Catalyzed Reduction of α,β-Unsaturated Ketones with Reagent-Controlled Chemoselectivity

Hall,

Rygus,

Boateng

2023

Synlett

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The use of an air-stable cationic hemiboronic acid catalyst for the chemoselective reduction of enones is described. By changing the identity and stoichiometry of the silane reducing agent, either the conjugate reduction products or the fully reduced products can be obtained in high selectivity. In contrast to analogous reactions catalyzed by air- and moisture-sensitive borane catalysts, the hemiboronic acid catalyzed protocol can be performed under ambient conditions. Profiling studies revealed that global reduction proceeds via a rapid initial 1,4-addition, followed by ketone deoxygenation with a rate that is highly silane-dependent.

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Recent Trends in Triarylborane Chemistry: Diversification of Structures and Reactivity via meta-Substitution of the Aryl Groups

Hoshimoto,

Sakuraba

2024

Synthesis

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This Short Review summarizes the synthesis and applications of triarylboranes (BAr3), including both homoleptic and heteroleptic species, with a focus on the modification of their electronic and structural properties via the introduction of meta-substituents with respect to the B atoms to their Ar groups. This approach constitutes a complementary alternative to conventional strategies for the design of BAr3, which are usually based on a modification of their ortho- and/or para-substituents. An initial analysis revealed that CH3 and F are the most common meta-substituents in hitherto reported BAr3 (apart from the H atom). Thus, an extensive exploration of other substituents, e.g., heavier halogens, longer or functionalized alkyl groups, and aryl groups, will increase our knowledge of the structure and reactivity of BAr3 and eventually lead to a range of new applications.1 Introduction2 Scope of this Review2.1 The Electronic and Steric Influence of meta-Substituents2.2 Molecular Transformations Mediated by meta-Substituted Boranes2.3 Other Examples of meta-Functionalization of BAr3 3 Conclusions and Perspectives

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Borane- and Silylium-Catalyzed Difunctionalization of Carbohydrates: 3,6-Anhydrosugar Enabled 1,6-Site Selectivity

Cited by 3 publications

References 37 publications

Resolving the Mechanistic Complexity in Triarylborane-Induced Conjugate Additions

Resolving the Mechanistic Complexity in Triarylborane-Induced Conjugate Additions

Hemiboronic Acid-Catalyzed Reduction of α,β-Unsaturated Ketones with Reagent-Controlled Chemoselectivity

Recent Trends in Triarylborane Chemistry: Diversification of Structures and Reactivity via meta-Substitution of the Aryl Groups

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